Conventionally, an air flow rate measurement device measuring an amount of an air drawn into an engine is known. The air flow rate measurement device includes a casing disposed in an intake air passage of the engine. The casing defines a bypass passage therein to take in a part of the air drawn in the engine, and an outlet of the bypass passage is open on a lateral surface of the casing. When the amount of the air drawn into the engine varies temporally, a pulsation flow is generated, and accordingly a speed of the air varies. A separated vortex generated when the speed of the air decreases tends to be larger than a separated vortex generated when the speed of the air increases. This is caused by a generation of an adverse pressure gradient in the decrease of the speed of the air. In the adverse pressure gradient, a pressure gradient is opposite to a flow direction of the air. When the pulsation flow is so large that the air flows in a backward flow direction, the separated vortex moves with the air flowing from a downstream side toward an upstream side of the casing in the backward flow direction. In this case, a part of the separated vortex generated on the downstream side of the casing enters the bypass passage, and accordingly a pressure loss increases. Consequently, a detection accuracy of a flow rate sensor may decrease.
Patent Document 1 (JP 4686455 B) discloses a technology capable of limiting a generation of the separated vortex. An air flow rate measurement device disclosed in Patent Document 1 includes an air discharge port on a lateral wall of a casing, a guide wall, and multiple protrusions on an outer surface of the guide wall. The guide wall is located downstream of the lateral wall having the discharge port, and the guide wall extends in approximately parallel to the lateral wall.
The technology disclosed in Patent Document 1 may not capable of limiting the separated vortex generated by an air flowing in a backward flow direction. An intake air pulsation is generated in an intake air passage of an engine according to opening and closing of a supply valve and a discharge valve. When the intake air pulsation is large, the air flows in a direction opposed to a forward flow direction. An airflow in the forward flow direction is referred to as a forward flow, and an airflow in the direction opposed to the forward flow direction is referred to as a backward flow. When the backward flow is generated in the intake air passage, the separated vortex is generated on the downstream side of the casing. The downstream side in the backward flow corresponds to an upstream side in the forward flow. However, the configuration of Patent Document 1 may not be capable of limiting the separated vortex generated when the air flows in the backward flow.